Rye-wheat mixed bread samples made with substitutions of 20, 40, 60 and 80% sourdough, and control made with the addition of naturally fermented raisin extract, were examined for quality characteristics such as pH, total titratable acidity, fermentation power of dough expansion, specific volume, baking loss, water activity, color, texture, external and internal surface appearance, and sensory qualities, in order to determine the optimal ratio of sourdough in the formulation. As the incubation time of sourdough increased, pH decreased, while total titratable acidity increased. The pH of rye-wheat mixed doughs decreased with increasing sourdough content, but total titratable acidity increased. The rye-wheat mixed breads prepared with substitutions of sourdough had higher pH and lower total titratable acidity than the control group. Fermentation power of dough expansion of rye-wheat mixed doughs increased with increasing incubation time. Specific volume was maximum on the 20% substitution sourdough. Baking loss was inferior with the 80% substitution sourdough. Water activity and lightness was minimum on the control group. The higher amounts of sourdough showed the higher tendency of lightness, redness and yellowness. In the texture characteristics, hardness, gumminess, and chewiness was minimum on the control group and tended to increase with higher substitution of sourdough. Substitution of sourdough showed decrease adhesiveness. Fracturability, cohesiveness, and resilience of all rye-wheat mixed breads were not significantly different. Springiness was maximum on the 20% substitution sourdough and minimum on the 40%. A side of loaf of rye-wheat mixed breads, except for 20%, had a poor break and shred. In sensory evaluation, as substitution amount of sourdough increased, the scores of color and consistency of crumb, uniformity of crumb pore, gumminess, and overall acceptability decreased; while the density of crumb pore, springiness of crumb, aroma of rye flour, sourness, and bitterness showed the reverse effect; the 20, 40, and 60% sourdough samples obtained fairly good scores. In conclusion, these results indicated that $20{\sim}60%$ of sourdough could be very useful as a substitute for baker's yeast in developing rye-wheat mixed bread.